In wireless communication technologies, synchronization procedures may be used for enabling efficient uplink (UL) radio transmissions of radio devices, also referred to as a UE (user equipment), to a base station. For example, in the LTE radio technology specified by 3GPP (3rd Generation Partnership Project) UL radio transmissions are based on an orthogonal modulation scheme called DFT-S-OFDM (Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing) that relies on tight synchronization of UL transmissions for maintaining orthogonality. Here, the synchronization aims at controlling the timing of UL transmissions of different UEs in such a way that the UL radio transmissions are time alignment upon reception by the base station. For this purpose, each of the UEs is configured with a timing advance (TA) value, which typically depends on the UE's position relative to the base station.
For enabling configuration of the TA value, the UEs may transmit UL signals to be used by the base station for determining the TA value, which is then indicated by the base station to the UE. These UL signals may for example be a random access preamble, a dedicated UL synchronization signal, a UL demodulation reference signal, or the like. Based on the time alignment provided by the TA value, the UE may then perform time-aligned DFT-S-OFDM radio transmissions, e.g., on a PUCCH (Physical UL Control Channel) or on a PUSCH (Physical UL Shared Channel). The time aligned DFT-S-OFDM radio transmissions may be used for conveying user plane data or UL control information.
One specific type of UL control information is HARQ (Hybrid Automatic Repeat Request) feedback for downlink (DL) radio transmissions from the base station to the UE. For the LTE radio technology, HARQ protocol functionalities are specified in 3GPP TS 36.321 V14.0.0. (2016-09). By means of the HARQ feedback, the UE can indicate to the base station whether a DL radio transmission from the base station was successfully decoded by the UE (by sending a HARQ ACK) or a DL radio transmission from the base station was not successfully decoded by the UE (by sending a HARQ NACK). Upon receiving a HARQ NACK, the bases station decide to perform a retransmission of the unsuccessful DL radio transmission. According to 3GPP TS 36.213 V14.0.0 (2016-09), the HARQ feedback is transmitted on the PUCCH or PUSCH.
Since the HARQ feedback is transmitted on the PUCCH or PUSCH, the UE thus needs to maintain its time alignment with the base station, which requires regular transmission of UL signals. This may cause undesirable energy consumption at the UE and also consumes radio resources for transmission of signals between the UE and the base station. If the UE loses its time alignment with the base station, transmissions may be delayed because before the UE can transmit HARQ feedback to the base station, it may first need to establish time alignment. On the other hand, DL radio transmissions are typically possible even if there is no valid time alignment of the UE.
Accordingly, there is a need for techniques which allow for efficient transmission of HARQ feedback for DL radio transmissions.